In the United States, the Federal Communications Commission (FCC) is charged with allocating and regulating the use of RF spectrum for radio and television broadcasting. Corresponding entities exist in other countries. The portions of the RF spectrum allocated for various types of terrestrial broadcasting are divided into channels having a defined bandwidth. For example, in the United States, the frequency-modulation (FM) broadcast band that extends from 88 to 108 MHz is divided into 100 channels with center frequencies starting at 88.1 MHz and ending at 107.9 MHz. Each channel has a bandwidth of 200 kHz. Similarly, the amplitude modulation (AM) broadcast band is divided into 10 kHz-wide channels extending from 530 kHz to 1710 kHz. Television (TV) broadcasting is accomplished using 6 MHz-wide channels in three bands extending from 54 MHz to 88 MHz (with a gap between 72 MHz and 76 MHz), 174 to 216 MHz and 470 to 806 MHz.
Available channels are allocated to broadcast stations geographically. Each broadcast station is assigned a service area to cover, typically a city or metropolitan area. The radiated power, antenna height and antenna radiation pattern of the broadcast station are specified such that the signal transmitted by the broadcast station provides a field strength greater than a threshold field strength throughout its service area.
In many areas, the availability of broadcast licenses is limited by the available spectrum. This is especially true in the FM and TV bands. Increasing the number of available channels by expanding the frequency range of the band is not practicable due to adjacent bands being used by other services and the tremendous number of existing receivers that can tune only the existing channels.
Accordingly, to increase the choice of broadcast programming available in many areas, what is needed is a way to increase the area density of broadcast stations in such areas without increasing the frequency ranges of the broadcast bands in which the broadcast stations operate and without an increased incidence of co-channel interference in the service areas of the broadcast stations.